Current communication transport networks are designed to transport mixed packet and Time-Division Multiplexed (TDM) structured services. The packet traffic typically comprises Internet Protocol (IP)/Multi-Protocol Label Switched (MPLS) traffic and Ethernet traffic and the TDM traffic can comprise voice or data traffic.
A transport network is architecturally considered as comprising a set of layers, as defined in the Open System Interconnection (OSI) reference model, with: Layer 1 (L1) being the physical layer, such as optics/Dense Wavelength Division Multiplexed (DWDM), Layer 2 (L2) being the Data Link Layer, using such protocols as Ethernet/Medium Access Control (MAC); and Layer 3 (L3) being the Network Layer, using such protocols as Internet Protocol (IP). Each of these three layers offers switching using the technology associated with the individual layers. For example, Layer 1 (L1) can have Synchronous Digital Hierarchy (SDH)/Synchronous Optical Networking (SONET), Optical Transport Network (OTN) or optical switching, Layer 2 (L2) can have Ethernet switches and Layer 3 (L3) can have routers.
FIG. 1 shows an example Layer 1 (L1) transport network comprising four nodes A, B, C, D. Layer 1 network equipment NE-A, NE-B, NE-C, NE-D is provided at each node. A Layer 2 switch and Layer 3 router is also provided at each node. A L1 NE forwards traffic to, and receives traffic from, a local L2 switch/L3 router at each node. A mix of traffic types is received at the ingress of node A. In FIG. 1 the traffic comprises: IP/MPLS traffic destined for Nodes B, C and D; Ethernet traffic destined for Nodes B, C and D; and TDM traffic destined for Nodes B, C and D. The mix of traffic types is mapped to L1 transport units (e.g. Optical Data Units (ODU) in the case of an OTN L1 transport network) and transported to node B. For example, TDM traffic can be mapped to a Layer 1 transport unit ODU-1#1, Ethernet traffic can be mapped to a Layer 1 transport unit ODU-1#2, and IP/MPLS traffic can be mapped to a Layer 1 transport unit ODU-1#3. Typically, the set of ODU-1 units are multiplexed into a higher level unit, such as ODU-2, and transported across the OTN to NE-B. At the L1 network equipment NE-B at node B, the ODU-2 is demultiplexed to the ODU-1 level. The L1 transport unit ODU-1#1 carries TDM traffic for various destinations. Traffic destined for Node B is removed from the ODU-1#1 and local traffic requiring transport across the OTN is added to ODU-1#1. The L1 transport units carrying Ethernet and IP/MPLS traffic ODU-1#2, ODU-1#3 are forwarded to a L2 switch/L3 router local to node B and unpacked. The L2 switch/L3 router inspects Layer 2/3 headers to decide if any of the traffic is destined for node B or one of the local nodes served by a network connected to node B. If so, the traffic egresses the network from the Layer 2 switch/Layer 3 router. All other traffic, which is destined for other nodes in the transport network, together with any locally generated traffic, is returned to the Layer 1 NE-B and forwarded to node C. Nodes C and D operate in a similar manner as node B.
The arrangement described above has several disadvantages. At each NE higher level Layer 1 transport units must be demultiplexed into lower level Layer 1 transport units. Also, at Node B and Node C L2 switches and L3 routers must process all L2 and L3 traffic, which puts significant demand on the L2 switches and L3 routers, with the consequence of very high bit rate interfaces, switching/routing capacity, power consumption, footprint and cost. There is also processing of the lower level Layer 1 data units carrying TDM traffic to groom TDM traffic to/from the local nodes.
The present invention seeks to provide an alternative way of operating a Layer 1 transport network, such as an Optical Transport Network (OTN).